The exhaust gases of internal combustion engines include carbon monoxide, unburned or partially burned hydrocarbons and nitrogen-oxides which all contribute to air pollution. It is necessary to monitor the exhaust products emitted from automotive internal combustion engines in order to decrease air pollution by eliminating as many of the polluting compounds as possible. It is necessary to remove as much carbon monoxide and unburned hydrocarbons as possible from the exhaust of these engines by converting carbon monoxide and hydrocarbons into their highest oxidation state, that is, in the case of carbon monoxide, into carbon dioxide and in the case of hydrocarbons into water, and then to convert the nitrogen-oxides into elementary nitrogen.
Removing polluting components from the exhaust of internal combustion engines can be effected by conducting the exhaust gases at a temperature above 600.degree. C. over a catalyst, so that the exhaust gases are subject to a post-combustion. The composition of the exhaust gases must be so arranged, however, that the relationship of air to fuel is approximately stoichiometric. Practically complete conversion to nonpolluting compounds is then possible. The stoichiometric relationship between air and fuel is characterized by a number .lambda. of unit (.lambda.=1). A lambda value equal to less than 1 means that no excess oxygen is present which exceeds the balance condition of the various respective actions which may take place; a lambda value greater than 1 means excess oxygen is present in the mixture; a lambda value exactly equal to 1 characterizes the state in which the exhaust gas changes from reducing to oxidizing.
Exhaust gases which are environmentally acceptable, as well as maximum use of fuel being supplied to an engine, better known as fuel efficiency, require the lambda value of the exhaust gases is approximately unity (.lambda.=1). Electro-chemical sensors when used to monitor and determine the oxygen content of an exhaust gas are exposed to the exhaust gases of an internal combustion engine. Control systems, responsive to output signals of the sensors, then react to adjust the relationship of air and fuel being supplied to the engine so that the air/fuel mixture will have a proper composition which, in turn, affects the exhaust gases emitted from the engine.
The oxygen sensors to which the present invention relates utilizes the principle of oxygen ion concentration and have ion conductive solid electrolytes. The solid electrolyte usually is in the shape of a tube, one end of which is closed; the closed end of the tube extending into the interior of an exhaust system so that the outside surface is exposed to exhaust gases. The outer surface, as well as the inner surface, of the electrolyte tube are coated, with an electron conductive layer, the outer surface including a catalyst. Each electron conductive layer, which may be in strip form, has a contact with which it is connected to an electrically conductive terminal portion. The terminal portions are usually so arranged that one terminal thereof is formed by the metal housing of the sensor which is secured to the exhaust system of an internal combustion engine. The other terminal is electrically connected to the inner postion of the oxygen sensor. The scientific principles upon which the solid electrolyte 0.sub.2 sensor operates may be found in U.S. Pat. No. Re. 28,792 entitled "Electrochemical Method for Separating 0.sub.2 From a Gas; Generating Electricity; Measuring 0.sub.2 Partial Pressure; and Fuel Cell" issued Apr. 27, 1976. The solid electrolyte most generally used in such sensors is zirconium dioxide which is a relatively weak structural material. In applying such a sensor to a heated environment such as the automotive exhaust system, it has become apparent that thermal stressing of the zirconium dioxide sensor body is a significant source of sensor failure. Further, external forces applied to an unprotected zirconium dioxide sensor body can cause cracking of the sensor body and/or stresses in the hermetic seals which result in failures (short useful life). The prior art inventors recognizing this problem provided a protective shield to surround the solid electrolyte extending from the oxygen sensor. An example of one such protective shield is shown in U.S. Pat. No. 3,835,012 entitled "Protective Shield for Oxygen Sensor" issued Sept. 10, 1974. The shield shown in this patent is not removable and inspection of the electrolyte before installation is not possible. Further, the shield, which protects the solid electrolyte, extends into the sensor housing between the solid electrolyte body and the sensor housing which mean that a hermetic seal must then be made between the housing, the protective shield, the electrolyte, and appropriate sealing gaskets. Accordingly, any forces applied to the shield will be transmitted to the hermetic seal and in many instances cause a failure of the seal. This is obviously disadvantageous since the solid electrolyte sensor works on the principle of the different oxygen partial pressures on opposite sides (isolated from each other) of the electrolyte. Therefore, it is essential that the hermetic seal be intact to isolate the reference gas inside the solid electrolyte tube from the exhaust gas outside the tube.
Accordingly, prior art oxygen sensors do not have removable shields around the solid electrolyte to allow inspection of the electrolyte; and in some instances, the shield extended into the housing and became part of the hermetic seal between the solid electrolyte and the housing, thereby requiring a more complex hermetic seal, and, when the shield was part of the hermetic seal, forces applied to the shield (dropping, tapping, etc.) where transmitted to the hermetic seal, causing failure.